Remove MCELFObjectTargetWriter::adjustFixupOffset hack

Now that PowerPC no longer uses adjustFixupOffset, and no other
back-end (ever?) did, we can remove the infrastructure itself
(incidentally addressing a FIXME to that effect).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181895 91177308-0d34-0410-b5e6-96231b3b80d8